An image forming device having a transfer unit that transfers the toner image formed on the image carrier onto transfer paper, such that the toner image can be transferred with good results regardless of fluctuations in the environment, by (a) returning to the feedback side of the power source, via a constant voltage element, part of the electric current that flows to (i) the transfer unit, to which a prescribed voltage is applied by the power source, and (ii) the guide member that supports the transfer paper, said electric current flowing to said guide member via said transfer paper, and by (b) controlling the voltage of the power source such that the voltage on the feedback side of the power source will be constant.